This invention relates to apparatus for converting single-phase a.c. power to polyphase a.c. power and more particularly to such apparatus which includes a rotary dynamoelectric machine.
Rotary phase converters are used to convert single-phase a.c. power to three-phase power. They are particularly useful in farm and some industrial settings where three-phase electrical machinery is needed but where three-phase utility power is unavailable. In use, such converters may supply three-phase power to one or many electrical machines, such as motors. Ideally, a rotary converter is capable of supplying this power indefinitely as long as the electrical load does not exceed the rated capacity of the converter.
A major problem with rotary converters is that as load conditions vary, electrical imbalances are created which result in unbalanced phase currents in the three-phase load. Depending upon the degree of this unbalance, excessive currents may exist in one or more of the load's phases. If the load is, for example, a three-phase motor, a current may be supplied to one of the motor windings which exceeds the maximum rated current carrying capacity for the winding. This causes overheating, particularly of the winding insulation. As a consequence, useful life of the motor or other electrical machine is reduced, necessitating more frequent machine replacement and increased expense to the owner or operator of a facility in which the machine is used.
There are several approaches to solving this problem but all are relatively expensive or otherwise capable of being improved. One approach is to derate the rotary converter so that it supplies power to only a fraction of its rated load capacity. This is unsatisfactory, however, since additional rotary converters are required to service the three-phase equipment and the capital investment for the facility is increased. Another approach is to add additional capacitance to the system, preferably on the load side of the converter. While this approach may improve the current balance somewhat, it is far from an adequate and economical solution to the problem since such additional capacitance increases both the cost and size of the converter. A third approach is shown in U.S. Pat. No. 4,079,446 and involves internally tapping the stator windings of the converter. This approach is a substantial improvement over the other two, but it too could be improved.